The present invention relates to an image sensor comprised of a number of light sensor circuits each representing a unit pixel, which circuit is capable of producing in a photoelectric converting element a photo current proportional to a quantity of incident light falling thereon, accumulating a corresponding amount of electric charge in a parasitic capacitor of the photoelectric converting element and outputting a sensor signal, and more specifically to an image sensor which is provided with means for preventing the effect of flickering of light from the light source.
There has been developed a conventional light sensor circuit having a wide dynamic range, which comprises a photo-diode PD operating as a photoelectric converting element for producing a photo current proportional to the quantity of incident light Ls falling thereon, a transistor having a logarithmic output characteristic in a weak inverse state for converting the photo current produced in the photodiode into a voltage signal by using its sub-threshold region characteristic (Japanese Laying-Open Patent Publications Nos. H05-219443 and H07-46481).
There also has been developed an image sensor comprising a number of the light sensor circuits each provided with a means for changing a drain voltage of the transistor to a value lower than a normal level value for a specified period to remove a charge accumulated in a parasitic capacitor of the photodiode to initialize the circuit (Japanese Laying-Open Patent Publication No. 2000-329616).
In the image sensor using a number of light sensor circuits each of which represents a unit pixel and works by producing in a photoelectric converting element a photo current proportional to a quantity of incident light falling thereon, accumulating a corresponding amount of electric charge in a parasitic capacitor of the photoelectric converting element and outputting a sensor signal, there may arise such a problem that pixels react to flickering of light of the light source such as a fluorescent lamp (brightness fluctuations in accordance with frequency of light of the light source), resulting in flickering in an image taken and displayed by the image sensor due to a difference between a flicker period of the light source and the video display rate. In this instance, the brightness of light of the light source at the time of reading sensor signals from the respective pixels becomes predominant in the image to be displayed. For example, a fluorescent lamp connected to a power source of a commercial frequency of 50 Hz flickers per period of 1/(2×50 Hz).
On the other hand, the video display rate is of 1/60 Hz for each frame. A least common multiple of the flicker period of the fluorescent lamp and the video display rate is a flicker period T of 0.05. Accordingly, flicker may be perceived in an image taken by the image sensor when it is displayed at 1/T=20 Hz on the display screen.
The image sensor using a number of light sensor circuits each of which represents a unit pixel and works by producing a photo current proportional to a quantity of incident light falling thereon, accumulating a corresponding amount of electric charge and outputting a sensor signal, the effect of flicker resulting from flicker of the light source can be prevented by adjusting the time of storing respective pixel signals. For example, it is possible to reduce the effect of flicker by adjusting amounts of accumulated charge of respective pixels to the same by using an electronic shutter to expose for 1/100 seconds light from the light source of the commercial frequency of 50 Hz. The provision of the electronic shutter may complicate the construction for preventing the flicker.
In Japanese Laid-Open Publications Nos. H05-115024 and 2001-86401, there is disclosed a conventional video camera using an array of solid-state light sensitive elements, which is provided with means for obtaining light source information by detecting a period and a phase of flicker in the light source and preventing the occurrence of flicker in an image by compensating the image signal based on the obtained light source information.
However, the means for detecting flicker conditions of light of the light source and compensating the image signal based on the light source information for removing the effect of flicker is complicate in construction. In order to sufficiently prevent flicker it is also needed to use a further complicated means for compensating image signals.
The image sensors of the type having a matrix of light sensor circuits using a number of light sensor circuits, each of which represents a unit pixel and works by producing in a photoelectric converting element a photo current proportional to a quantity of incident light falling thereon, accumulating a corresponding amount of electric charge in a parasitic capacitor of the photoelectric converting element and outputting a sensor signal, are further provided with means for preventing the occurrence of flicker resulting from flicker of the light source in an image taken and displayed on a display screen by controlling an electronic shutter provided for evenly storing all electric charges of respective pixels or by detecting flicker condition of the light source and compensating image sensor signals according to the detected flicker condition information. All the flicker preventing means considerably complicate the construction of the image sensors.